bpo-37337: Fix a GCC 9 warning in Objects/descrobject.c

[ZackerySpytz]

Commit b1263d5 causes GCC 9.1.0 to
give a warning in Objects/descrobject.c.

Objects/descrobject.c:1050:19: warning: cast between incompatible function types from ‘PyObject * (*)(mappingproxyobject *, PyObject * const*, Py_ssize_t)’ {aka ‘struct _object * (*)(struct <anonymous> *, struct _object * const*, long int)’} to ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *)’} [-Wcast-function-type]
 1050 |     {"get",       (PyCFunction)mappingproxy_get,        METH_FASTCALL,
      |                   ^

https://bugs.python.org/issue37337

[ZackerySpytz]

@methane

[ZackerySpytz]

Hi, @methane. Can you please merge this PR?

[methane]

The cast seems too complex to just avoid the warning. But I'm not sure there is a better way.
When I have spare time, I'll try to find simpler ways to fix this. But I don't have spare time now.
That's why I don't merge this.

If you want to this merged, please provide all casts you tried and demonstrate this is the simplest way to fix the warning.

[jdemeyer]

This is the standard way to avoid those warnings, there are many places in CPython with casts like that. It's also documented to do this: https://docs.python.org/3.8/extending/extending.html#keyword-parameters-for-extension-functions

[jdemeyer]

See bpo-33012 for the discussion about those casts.

As far as I know, this is indeed the simplest way to cast between incompatible function pointers which does not generate warnings with GCC 9 and which does not rely on undefined behaviour and which is compatible with C++.

[methane]

OK, thank you.

[miss-islington]

Thanks @ZackerySpytz for the PR, and @methane for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[miss-islington]

Sorry, @ZackerySpytz and @methane, I could not cleanly backport this to 3.8 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 43d564c18c97421f73025ac3132a194975c76bd6 3.8